Solve for $y$ : $9 + y = 15$
Subtract $9$ from both sides: $ \begin{eqnarray} \\ 9 + y &=& 15 \\ \\ {-9} && {-9} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{9 + y} &\hphantom{=}& \hphantom{15} \\ y &=& 15 {- 9} \\ \end{eqnarray} $ Simplify. $ y = 6$